Fix potential typo in hole rule?

Does the number of arguments need to be the same as the number of premises? If so this PR can be deleted otherwise, it fixes this typo.

requested review from @hbarbosa and @hjsc

assigned to @Lachnitt

changed title from Fix potential typo? to Fix potential typo in hole rule?

I don't see why the number of arguments and premises should agree. Thanks for fixing this!

mentioned in commit f2ee49ab

merged